Circuit for operating an electric motor

ABSTRACT

A circuit for operating an electric motor ( 11 ) includes a monitoring arrangement ( 24 ) for ascertaining an increased load state of the electric motor ( 11 ). The monitoring arrangement ( 24 ) compares an rpm difference (N D ) between a predetermined nominal rpm value (N SOLL ) and an actual rpm value signal (N IST ) with a predetermined limit value. If the limit value is exceeded, the monitoring arrangement ( 24 ) emits an overload signal ( 25 ) that reduces the nominal rpm value (N SOLL ) or a control variable (S N ) supplied to a drive stage ( 18 ), or drops it to zero. The circuit is particularly suited for use with a fan, in which instance the ventilator ( 22 ) cools the electric motor ( 11 ) and/or the drive stage ( 18 ).

BACKGROUND OF THE INVENTION

Related Art

The invention is based on a circuit for operating an electric motor, ofthe type having an arrangement that produces a signal serving as ameasure for the rpm of electric motor and having a monitoringarrangement for detecting an increased load state of the electric motor.DE-PS 30 34 118 discloses a generic circuit that has a sensor whichproduces a signal serving as a measure for the rpm of the electricmotor. The known circuit includes a monitoring arrangement forascertaining an increased load state of the electric motor; thearrangement monitors rpm, comparing the actual rpm determined from thesensor signal to a predetermined limit value. The limit value is derivedfrom an initial actual rpm value established after a predetermined timefollowing startup of the known circuit. The known circuit for operatingan electric motor is used in electronic monitoring of an opening andclosing process of electrically-operated motor vehicle parts. The limitvalue is tailored to the task of detecting jammed objects or body parts.The known circuit is not provided with an increased load state of theelectric motor, or of a provided end stage, that can lead to a thermaloverload of these parts.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a circuit for operating amotor that in particular assures the protection of an end or drive stageand/or the electric motor against thermal overload.

The object is accomplished by a circuit of the above mentioned typewhich is characterized in that the monitoring arrangement compares anrpm difference, between a predetermined nominal rpm value and the actualrpm determined from the actual rpm signal, to a predetermined limitvalue, and emits an overload signal if the limit value is exceeded.

A monitoring arrangement provided in accordance with the inventioncompares the difference between a provided nominal rpm value and anactual rpm value determined from a sensor signal. If the limit value isexceeded, the monitoring arrangement emits an overload signal.

The concept underlying the circuit of the invention is that, in thesteady operating state, the difference between the predetermined nominalrpm value and the detected actual rpm value either lies within specificlimits as a function of a predetermined rpm regulator or is zero.Sluggishness of the electric motor or of a part driven by the motor, aswell as complete blockage, cause the limit value to be exceeded, andthus lead to triggering of the overload signal.

An advantage of the circuit of the invention for operating an electricmotor is that an increased load state or complete blockage can berecognized with electrical signals that can be detected with simplemeans. No further sensors, such as a temperature sensor, are necessary.

The circuit of the invention for operating an electric motor isparticularly suited for use in a fan, in which instance the fan is usedto cool an electric motor and/or the drive stage simultaneously.

In comparison to establishing a limit value based on a predetermined rpmvalue, the circuit of the invention for operating an electric motor hasthe advantage that both the electric motor and the drive stage only needto be designed for low thermal stress.

A particularly advantageous modification of the circuit of the inventionprovides that the limit value for the rpm difference, which is providedin the monitoring arrangement, is determined as a function of thepredetermined nominal rpm value. With this measure, the limit value canbe adapted to different operating relationships.

An advantageous embodiment provides that the predetermined nominal rpmvalue is reduced when the overload signal is emitted. With anotheradvantageous measure, when the overload signal occurs, a controlvariable of a controlled voltage source is reduced, which source isinstrumental in the actuation of the drive stage. Implementing one ofthese measures ensures that, despite an increased load state, theelectric motor can continue to be operated reliably without the threatof thermal destruction.

In a particularly advantageous modification, the monitoring arrangementdoes not emit the overload signal until a predeterminable period of timehas passed. The predeterminable time is a delay time that takes intoaccount the run-up of the electric motor, for example starting from theoff state. The unsteady state that occurs during run-up and could leadto erroneous generation of the overload signal can be kept in check withthis measure.

Another advantageous modification provides that the overload signal issuppressed following a predetermined time. If the electric motor isfurther overloaded, the overload signal is emitted again. In thisoperation, periodic attempts are made to turn on the electric motor. Thetimes during which the overload signal occurs or is suppressed areadvantageously measured such that, despite an overload or completeblockage of the electric motor, the average power is limited to a valuethat does not represent a danger of thermal overload of the electricmotor and/or the drive stage.

Further advantageous embodiments and advantageous modificationsdiscussed in the description below.

BRIEF DESCRIPTION OF THE DRAWING

The drawing FIGURE shows a block diagram of a circuit of the inventionfor operating an electric motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows an rpm-regulating circuit 10 that regulates the rpm ofan electric motor 11 at a predetermined command or rpm value N_(SOLL).The nominal rpm value N_(SOLL), is produced by a command or nominalvalue generator 12. The nominal value generator 12 transmits the nominalrpm value N_(SOLL) to a nominal value reducer 13, which conducts thenominal rpm value N_(SOLL) further, as a limited nominal rpm valueN′_(SOLL), to an adder 14. The adder 14 forms an rpm difference N_(D)between the limited nominal rpm value N′_(SOLL) and an actual rpm valuesignal N_(IST). The actual rpm value signal N_(IST) is a measure for therpm of the electric motor 11 that is produced by an rpm sensor 15.

The adder 14 transmits the rpm difference N_(D) to a regulator 16 thatdetermines a control variable S_(N). The regulator 16 includes a controlvariable limiter 17. The control variable S_(N) enters an end stage ordrive stage 18 connected to a heat sink or cooling body 19. The drivestage 18 connects the electric motor 11 to an energy source, not shownin detail, by way of a first connection 20 and a second connection 21.The drive stage 18 is controlled with the control variable S_(N) in sucha way that the rpm of the electric motor 11 corresponds to thepredetermined nominal rpm value N_(SOLL).

A current flowing through the electric motor 11 leads to a motor voltageU_(M), which corresponds to the voltage drop at an armature resistanceR_(A), to which an electric-motor countervoltage (EMK) is added.

The electric motor 11 actuates a ventilator 22 that emits a first aircurrent 23 a in the direction of the electric motor 11 and a second aircurrent 23 b in the direction of the cooling body 19.

The rpm difference N_(D) and the nominal rpm value N_(SOLL) are suppliedto a monitoring arrangement 24 that produces an overload signal 25,which is transmitted to both the nominal value reducer 13 and thecontrol variable limiter 17. The monitoring arrangement 24 includes afirst, a second and a third timer 26, 27 and 28, respectively, and alimit value generator 29.

The illustrated circuit of the invention for operating the electricmotor 11 operates as follows:

The rpm-regulating circuit 10 regulates the rpm of the electric motor 11in such a way that the rpm difference N_(IST) between the nominal rpmvalue N_(SOLL) produced by the nominal value generator 12 and the actualrpm value signal N_(IST) produced by the rpm sensor 15 is as low aspossible, or is even zero. To regulate the rpm, the rpm-regulatingcircuit 10 includes the adder 14, which determines the rpm differenceN_(D), as a deviation, from the actual rpm value signal N_(IST) and thelimited nominal rpm value N′_(SOLL). The regulator 16 establishes thecontrol variable S_(N) as a function of the rpm difference N_(d). Theregulator 16 can be configured in accordance with the extensivebackground literature on regulating technology, for example as aproportional or proportional-plus-integral regulator. Both analog anddigital regulators are suitable. The control variable S_(N) can be ananalog signal that is supplied to the drive stage 18. The controlvariable S_(N) is preferably a digital, pulse-width-modulated signalthat effects a clocked operation in the drive stage 18. In this clockedoperation, the electric motor 11 is connected to the energy source notshown in detail, or completely shut off, in rapid temporal sequence. Theresult is an average voltage U_(M), that leads to a specific motorcurrent as a function of the load set by the ventilator 22. The drivestage 18 can be characterized as a controlled voltage source that setsthe motor voltage U_(M) variably at a value at which the rpm differenceN_(D) is as low as possible or is zero. The clocked operation of thedrive stage 18 with the pulse width control permits the drive stage 18to be designed for a switching operation that results in minimal energyloss in the drive stage 18 that must be dissipated via the cooling body19. The cooling body 19 is either separate and connected to the endstage 18, or a specially-worked component of the drive stage 18.

The rpm of the electric motor 11 is detected by the rpm sensor 15. It isemphasized that the rpm of the ventilator 22 or the rpm of a drive notshown in detail can also be detected instead of the motor rpm. The onlyessential point is that the rpm sensor 15 produces the actual rpm valuesignal N_(IST) as a measure for the rpm of the electric motor 11.

The rpm sensor 15 may operate on, for example, an optoelectronic basis.A light barrier is suitable. The rpm sensor 15 preferably operates on amagnetic basis, for example detecting a magnetic field. Hall sensors,magnetoresistive elements or inductive receivers are examples ofsuitable devices. The detection of commutations signal components in themotor current is also particularly suited. The rpm is preferablydetected indirectly through the evaluation of the operating data of theelectric motor 11. The rpm is proportional to the inducedcountervoltage, which in turn can be determined from the motor terminalvoltage and the motor current with a known internal resistance.

A feature of the circuit is the monitoring arrangement 24 that comparesthe rpm difference N_(D) to the limit value produced by the limit valuegenerator 29. If the rpm difference N_(D) exceeds the limit value, themonitoring arrangement 24 emits the overload signal 25. The overloadsignal 25 can trigger a warning signal, for example. In the illustratedembodiment, the overload signal 25 can, for example, lower the nominalrpm value N_(SOLL)predetermined by the nominal value generator 12 by apredetermined amount, or even make it zero, in the nominal value reducer13. The lowered nominal rpm value N_(SOLL) is characterized as a limitednominal rpm value N′_(SOLL) that is supplied to the adder 14. It is alsopossible for the overload signal 25 to lower, for example, the controlvariable S_(N) of the regulator 16 by a predetermined amount or to zerowith the control variable limiter 17 included in the regulator 16.

A particularly advantageous embodiment provides that the limit valueproduced by the limit value generator 29 is established as a function ofthe predetermined nominal rpm value N_(SOLL). Therefore, in addition tothe rpm difference N_(D), the nominal rpm value N_(SOLL) is supplied tothe monitoring arrangement 24 as an input signal. With a high nominalrpm value N_(SOLL) the limit value is preferably established to be lowerthan with a lower nominal rpm value N_(SOLL).

Another advantageous embodiment includes the first timer 26 in themonitoring arrangement 24. The first timer 26 is set at a time thatcorresponds to the run-up of the electric motor 11, for example fromstandstill to the predetermined normal-rating operation, at which thenominal rpm value N_(SOLL) is reached. During the time predetermined bythe first timer 26, emission of the overload signal 25 is suppressed.The first timer 26 recognizes the turning on of the electric motor 11,for example through a change in the presetting of the nominal rpm valueN_(SOLL).

The second and third timers 27, 28 permit a periodic, recurring attemptto start up the electric motor 11 despite an ascertained overload stateor increased-load state. The second timer 27 starts, for example,simultaneously with the occurrence of the overload signal 25. The timeset by the second timer 27 therefore establishes the shutoff time of theelectric motor 11 or the time of reduced power. After the timepredetermined by the second timer 27 has expired, the overload signal 25is suppressed during the time predetermined by the third timer 28.Therefore, during the time predetermined by the third timer 28, anattempt is made to resume normal-rating operation of the electric motor11 with the predetermined nominal rpm value N_(SOLL). A temporaryblocked state or temporary sluggishness is identified with this measure,and has no further effects on continued operation. Because the thirdtimer 28 predetermines a turn-on time and the second timer 27predetermines a shutoff time or the time of reduced power, it ispossible to keep thermal overload of both the electric motor 11 thedrive stage 18 cooled by the cooling body 19 within preset limits, evenif overload occurs during long-term unsteady operation. A suitablesetting can be selected, for example, in that the average current I_(M)flowing through the electric motor 11 is selected to meet the followingcondition:

I _(M) =T _(ein)/(T _(ein) +T _(aus))×(U _(Mmax) /R _(A))

where T_(ein) is the turn-on time, T_(aus) is the shutoff time andU_(Mmax) is the maximum motor voltage. Thermal overload is precludedwith an average motor current set in this way.

A preferred use of the circuit of the invention for operating anelectric motor 11 is in a fan, preferably one disposed in a motorvehicle and serving to ventilate the interior. A modification of the usein a fan in accordance with the invention provides that the drive stage18 and/or the electric motor 11 is or are cooled by the air current 23 a23 b of the ventilator 22 itself. With this measure, it is possible toprovide an inexpensive end stage 18 whose permissible continuous energyloss can have a lower value than a drive stage 18 that does not benefitfrom additional cooling. A crucial feature of this modification,therefore, is that, when the electric motor 11 is sluggish or completeblockage exists, the motor current in the drive stage 18 and thereforethe energy loss of the drive stage are reduced after the additionalcooling by the ventilator 22 has ended.

A transistor, preferably a field-effect transistor, is particularlysuited to be the drive stage 18. In a bipolar transistor, the controlvariable S_(N) is a control current that flows into the base duringanalog operation. In a field-effect transistor, the control variableS_(N) is a control voltage that impinges upon the gate. In all cases, apulse-width-modulated, digital control variable S_(N) is provided inclocked operation; this variable operates the drive stage 18 as acontrolled voltage source that predetermines the average motor voltageU_(M).

In principle, it is also possible to operate the circuit of theinvention without an rpm-regulating circuit 10. Instead of rpmregulation, an rpm control is provided, with the monitoring arrangement24 detecting the rpm difference N_(D) in controlled operation, just asin the former case.

What is claimed is:
 1. An improved circuit for operating a DC electricmotor, the circuit having an arrangement that produces a signal servingas a measure for the rpm of the electric motor and having a monitoringarrangement for detecting an increased load state of the electric motor,wherein the improvement comprises: the monitoring arrangement (24)compares an rpm difference (N_(D)) between a predetermined nominal rpmvalue (N_(SOLL)) and the actual rpm determined from the actual rpm valuesignal (N_(IST)) to a predetermined limit value, and emits an overloadsignal (25) if the limit value is exceeded.
 2. A circuit according toclaim 1, wherein the limit value is a function of the predeterminednominal rpm value (N_(SOLL)).
 3. A circuit according to claim 1, whereinthe monitoring arrangement (24) includes a first timer (26) that takesinto account a run-up process of the electric motor (11) with respect tosetting of the nominal rpm value (N_(SOLL)) by suppressing the overloadsignal (25).
 4. A circuit according to claim 1, wherein the arrangementthat produces a signal serving as a measure for the rpm includes an rpmsensor.
 5. A circuit according to claim 1, wherein the arrangement thatproduces a signal serving as a measure for the rpm detects the motorcurrent and the motor terminal voltage, and determines the rpm from thedetected motor current and the motor terminal voltage.
 6. A circuitaccording to claim 1, in combination with a fan that is driven by theelectric motor (11), wherein the circuit further comprises a drive stage(18) for driving the electric motor (11) and the fan cools at least oneof the electric motor (11) and the drive stage (18) using air currents.7. An improved circuit for operating a DC electric motor, the circuithaving an arrangement that produces a signal serving as a measure forthe rpm of the electric motor and having a monitoring arrangement fordetecting an increased load state of the electric motor, wherein theimprovement comprises: the monitoring arrangement (24) compares an rpmdifference (N_(D)) between a predetermined nominal rpm value (N_(SOLL))and the actual rpm determined from the actual rpm value signal (N_(IST))to a predetermined limit value, and emits an overload signal (25) if thelimit value is exceeded; and the current further comprises a nominalvalue reducer (13) that receives the overload signal (25) and thatlowers the nominal rpm value (N_(SOLL)) to a limited nominal rpm value(N′_(SOLL)) in response to the overload signal (25).
 8. An improvedcircuit for operating a DC electric motor, the circuit having anarrangement that produces a signal serving as a measure for the rpm ofthe electric motor and having a drive stage for driving the electricmotor in response to a control variable, the circuit additionally havinga monitoring arrangement for detecting an increased load state of theelectric motor, wherein the improvement comprises: the monitoringarrangement (24) compares an rpm difference (N_(D)) between apredetermined nominal rpm value (N_(SOLL)) and the actual rpm determinedfrom the actual rpm value signal (N_(IST)) to a predetermined limitvalue, and emits an overload signal (25) if the limit value is exceeded;and the circuit further comprises a regulator (16) which generates thecontrol variable (S_(N)) as a function of the rpm difference (N_(D)),the regulator (16) including a control variable limiter (17) thatreceives the overload signal (25) and that reduces the control variable(S_(N)) supplied to the drive stage (18) to a predetermined value inresponse to the overload signal (25).
 9. An improved circuit foroperating an electric motor, the circuit having an arrangement thatproduces a signal serving as a measure for the rpm of the electric motorand having a monitoring arrangement for detecting an increased loadstate of the electric motor, wherein the improvement comprises: themonitoring arrangement (24) compares an rpm difference (N_(D)) between apredetermined nominal rpm value (N_(SOLL)) and the actual rpm determinedfrom the actual rpm value signal (N_(IST)) to a predetermined limitvalue, and emits an overload signal (25) if the limit value is exceeded,and the monitoring arrangement (24) includes a timer (27) that limitsthe duration of the overload signal (25) to a time (T_(aus))predetermined by the timer (27), and a further timer (28) thatpredetermines a time (T_(ein)) during which the overload signal (25) isthen suppressed.
 10. A circuit according to claim 9, further comprisinga drive stage (18) for driving the electric motor (11), and wherein thetwo times (T_(aus), T_(ein)) predetermined by the two timers (27, 28)are measured so as to preclude thermal overload of at least one of theelectric motor (11) and the drive stage (18).
 11. A circuit foroperating a motor in response to a nominal rpm signal, comprising: meansfor producing an actual rpm signal which serves as a measure of the rpmof the motor; means for generating a control variable signal which is afunction of the difference between the nominal and actual rpm signals;means for comparing the difference between the nominal and actual rpmsignals to a limit value and generating an overload signal if thedifference exceeds the limit value; means for reducing the controlvariable signal if the overload signal appears; and means for drivingthe motor in response to the control variable signal.
 12. A circuitaccording to claim 11, wherein the limit value is a function of thenominal rpm signal.
 13. A circuit according to claim 11, wherein themeans for generating a control variable signal comprises an adder whichreceives the nominal and actual rpm signals and generates a differentsignal from them, and a nominal value reducer which reduces the nominalrpm signal before it is received by the adder if the overload signalappears.
 14. A circuit according to claim 11, wherein the means forgenerating a control variable signal comprises an adder which receivesthe nominal and actual rpm signals and generates a difference signalfrom then, and a regulator which generates the control variable signalfrom the difference signal, the regulator including a control variablelimiter which limits the magnitude of the control variable signal if theoverload signal appears.
 15. A circuit according to claim 11, whereinthe means for generating an overload signal comprises a timer whichsuppresses the overload signal during run-up of the motor.
 16. A circuitaccording to claim 15, wherein the means for generating an overloadsignal further comprises another timer which limits the duration of theoverload signal and an additional timer which then times an intervalduring which the overload signal is suppressed.
 17. A circuit accordingto claim 11, wherein the motor is a DC motor, wherein the controlvariable signal is a pulse-width modulated signal, and wherein the meansfor driving the motor selectively connects the motor to a power sourcein response to the control variable speed.
 18. A circuit according toclaim 11, wherein the motor is a DC motor.